Printing machines have been developed in which the transfer of ink to a web of material, such as paper, is not only provided by direct transfer of ink from an engraved cylinder, but also by electrostatic attraction of ink to the web. This is accomplished by passing the web through a nip region where the underside of the web is contacted by the engraved cylinder as it is being pressed down against the cylinder by an impression roller. Transfer of the ink is assisted by applying an electrical voltage to the impression roller in the nip region in a pattern determined by the engraved cylinder.
Impression rollers have typically been formed around a steel core that is grounded and insulated from a semiconductive layer that is formed on the exterior of the roller to conduct current to the nip region. The materials used in the outer layer of the impression roller exhibits the resistance of a semiconductor, and is also resilient, which allows for flattening of the outer surface of the impression roller in the nip region.
In the Knopf, et. al U.S. Pat. No. 5,044,275 a new electrostatic printing assist system is disclosed in which a rotary step-up transformer is attached to one end of the impression roller.
The new system consists of a low voltage D.C. power supply with appropriate controls for voltage, current, and safety; a step-up transformer, and a contact assembly to the impression roller. The design is unique in that the rotary step-up transformer is doughnut shaped and split into two pieces. The input side of the transformer is stationary and mounted to the gravure printing press. The output side is attached to the impression roller. A low voltage of about 100 to 200 volts is brought to the output transformer. As a safety feature, the high voltage (approximately a factor of 10 higher) exists only on the impression roller. The high voltage is rectified and filtered inside the output assembly on the impression roller, then fed by a special contact assembly to the middle layer of a three-layer impression roller.
In a normal three-layer impression roller, the first layer (base layer) on the steel core is an electrical insulator, the next layer (middle layer) is a low resistance material that is usually 10 4 ohm-cm or less in volume resistivity, and the top layer (surface layer) is a semiconductor in the 10 7 to 10 8 ohm-cm range. The middle layer which should be as thin as practical for roller stability provides a low resistance path and is usually encapsulated within the other two layers as a safety measure. The new system requires that the middle layer be exposed on one end of the roller so it can mate with the contact assembly.
The manufacturer's dimensional specifications for the base and middle layers on the end of the impression roller with the middle layer exposed are difficult to meet with normal rubber roller fabrication techniques. According to the manufacturer specifications, the base layer should be about 4.0 mm thick +/-0.1 mm, and the middle layer about 3.0 mm +/-0.1 mm. The ring formed by the middle conductive layer must also be concentric with the axis of the roller core. These dimensions were believed to be necessary to insure proper alignment and connection to the contact assembly, and that the middle layer is both electrically isolated from the core and contained within the outer dimension of the contact assembly housing. However, roller stability and life could be improved by making the intermediate conductive layer thinner.
In normal roller fabrication, the rubber is applied to the core in layers of calendered rubber each having a thickness of about one to two millimeters. Because the rubber is applied as a spiral layer, it is somewhat eccentric around the axis of the core. Also, when the roller is vulcanized and there is a certain amount of rubber flow (movement) that takes place that further distorts the thickness, uniformity, and concentricity of the different layers. As a result, it is difficult to prepare a conventional impression roller with a relatively thin intermediate conductive layer that still meets the requirements for use in the Knopf et al. patented system.